In actual practice, the above-mentioned emergency cooling system consists of a first sprinkler system comprising a plurality of nozzles or sprinklers mounted in the upper part of the reactor and adapted to spray large amounts of water on the fuel rods in order to cool these when there is an emergency. The plant further includes a second sprinkler system comprising a plurality of nozzles or sprinklers which, like those of the first system, take their water from the condensation pool in the containment, but which are mounted outside the reactor proper and are adapted to sprinkle the gas phase in the containment in order to reduce any remaining excess pressure therein as well as to cool conduits or other components found inside the containment but outside the reactor itself. In both instances, it is of great importance that the water supplied to the nozzles is free from all sorts of impurities, such as fibres, grains and particles, that might clog the nozzles. Naturally, this is especially important in the emergency cooling system, which has to be absolutely reliable. Many of the components mounted inside the containment, such as the conduits, are wholly or partly heat insulated. In most of today's nuclear power plants, this insulation is made up of fibres of mineral wool, which constitute an element of risk with regard to the two sprinkler systems, in that unintentionally released fibres may clog the nozzles if reaching the sprinkler systems. For this reason, nuclear power plants have been equipped with strainers of the type stated by way of introduction.
In actual practice, it takes about 5-10 min to back-flush a conventional strainer which is contaminated with a fibre mat tending to clog the strainer holes. It was previously held that the strainers could operate for at least 10 h without any need of back-flushing. However, real-life incidents have shown that this estimated minimum operating time is too long. In functional tests, it has happened that discharged steam has entrained mineral-wool insulation, which has dropped into the condensation pool and clogged the-strainers even after about 30 min. Back-flushing, which takes 5-10 min, is not a critical operation 10 h after a possible reactor trip, since the decay power of the reactor core then has been considerably reduced, as has the need for cooling. However, if back-flushing is required after less than 1 h, the need for cooling of the core is still considerable, and an interruption of the water supply to the emergency cooling system for 5-10 min therefore is unacceptable for reasons of safety.
One reason why back-flushing of the strainer takes such a comparatively long time is that the fibres accumulating on the outside of the strainer wall form a continuous, circumferential mat or cake in which they are closely intertwined. The wash water flowed from the inside and radially outwards through the perforations in the strainer wall does not provoke any immediate release of the entire mat, but initially merely stretches the mat while breaking up the fibre structure, so that individual fibres are successively released and removed from the mat. It is only after considerable hydromechanical action that the mat grows weaker and is successively divided into chunks that leave the strainer wall. Another reason is that the wash water is allowed to flow into the strainer along a substantially axial path, which entails considerable local variations of the flow intensity in the different perforations in the strainer wall. To be more precise, the flow concentrates in the upper end of the housing that is opposite to the inlet end. This means that the fibre mat will be released in chunks, beginning at the upper part of the housing and proceeding downwards. In this late phase, considerable amounts of wash water will flow through the upper, already uncovered perforations without affecting the lower perforations still coated with fibres.